Skip to Content

What Size Wire for a 40-Gallon Electric Water Heater? [Explained]

Measuring the ideal size of wires for a 40-gallon electric water heater is very important. Unmatched wires can lead to short circuits.

You don’t want that, do you? 

So now the question comes, what size wire for a 40-gallon electric water heater?

An average 40-gallon electric water heater requires 12 gauges of wire. You cannot use a 6 gauge or 10 gauge wire in that case. The current will overflow and the wire will start melting. However, keep in mind that the size varies according to a few factors.

Let’s find out what we should consider when choosing a heater wire!

Why The Sizes of Wires Vary in Different Water Heaters

Water heaters come in different sizes and capacities. Considering the size of gallons, the size of wires also differs.

What Size Wire for a 40-Gallon Electric Water Heater? [Explained] 1

For example, a 40-gallon electric water heater’s ideal wire size is 12 gauge. 50 Gallon electric water heaters require 6 gauges of wire.

The higher the gallon size the smaller the wire.

The wiring may also depend on whether the water heater is vertically installed or horizontally.

Amperage is also a part here. For example, A 20-amp hot water heater’s ideal wire size is 12-gauge. But, a 25-amp hot water heater’s wire size is 10-gauge wire. 

How to Calculate A 40-Gallon Water Heaters Wire Size

What Size Wire for a 40-Gallon Electric Water Heater? [Explained] 2

Now you can’t just choose any wire size for a 40-gallon heater. First of all, you need to collect some data to measure it. Such as:

  • Type of wire (copper or aluminum)
  • Circuit phase
  • Circuit voltage
  • One half of the total circuit length
  • Total circuit amperage

An average 40-gallon electric water heater runs at 240 volts and a wattage of 2500 W. Hence, calculating the amperage 2500/240, we can deduce the amperage is 18.75 A. It can approximately be calculated as 20 A.

One-half of the total circuit is 80 meters and has a single circuit phase.    

Now, you can measure a 40-gallon wire size using a calculator. So, you can use a 12 gauge copper wire for this.

You can calculate the wire size for your heater by following the method above.

Looking for the best wire to use in an electric heater? You’re on the right track!

We have some suggestions for you:

Product 1
Product 2

Now you have your wire, let’s find out why it’s important to have the perfect size wire. 

Why Matching The Wire Gauge Is Important For An Electric Heater

If you’re changing the wire for your heater, you’ve to understand why matching it is very crucial.   

It reduces the chances of heating the wire. On the other hand, if you use unmatched wire, it will lead to short circuits. 

How? Here’s why-

Suppose you’re using a 40-gallon water heater on a 30-amp circuit with 6 gauge wire

A 220 volts heater passes more current than the wires can hold off. It starts melting the surrounding materials, similar to when your shower cable heats up!

The danger level is very severe. One misalignment could even kill someone.

Things to Keep in Mind While Replacing Wires of An Electric Heater

Before replacing any water heater wiring, you need to get a permit. An inspector reviews if all the electrical and plumbing connections are connected properly or not. 

If you’re going to replace the wire, you need to follow some instructions:

  • The circuit wiring is included with a 40-amp double-pole breaker. It also includes 10-2 non-metallic (NM) cables. 
  • The black circuit wire should connect to the lead of the black wire. The white circuit wire should connect the lead of the red or white wire. 
  • At the breaker box, the white circuit wire should be wrapped with black or red electrical tape. It symbolizes which one is a “hot” wire and which is the neutral wire? 
  • A 240-volt circuit passes current in both wires (black and white). So, connect the circuit ground wire with the green ground screw of the water heater. 

Quick Tip: Make sure you’re using the precise size breaker otherwise you’ll damage your heater. If all else fails, call a professional mechanic to fix this problem.


Question: Can I replace my hot water heater myself?

Answer: If you have experience, you can replace a hot water heater by yourself. But no matter how well experienced you are, you have to take permission for it. Also, before replacing them, follow the dos and don’ts.

Question: Can I put a 40A breaker on a 10 gauge wire?

Answer: You cannot put a 40A breaker on a 10 gauge wire. You need to use an 8 gauge wire for a 40A circuit breaker. You can use 10 gauge wire in a 30A breaker. 

Question: How many amps are 4500 watts?

Answer: 4500 watts is equal to 18.75 amps. This is for an electric circuit of 240 volts. 

Bottom Line

That’s it! You can now measure what size wire for a 40-gallon electric water heater. 

If you want to replace the wire of your heater, follow the instructions given in the article. 

That’s all for today! Hope we’ve got rid of any confusion regarding what to do.

Till then, stay safe and sound!